This invention relates to a process for obtaining a multilayer metallization of the back of a semiconductor substrate.
Numerous semiconductor electron devices use the back of the substrate, on which they are constructed, as an electrical terminal. This area must therefore be metallized in order to enable the electrical connection and the M-S (Metal Semiconductor) contact must have a sufficiently low specific contact resistance as to ensure that no appreciable voltage drops occur when the current passes through (i.e.- an ohmic contact).
In the majority of cases, the metallization on the back must also ensure the possibility of welding the latter onto a metal support which forms part of the package of the finished device and also acts as a heatsink. In order to simultaneously guarantee these characteristics, very often several layers of metal are deposited on the back, with the first layer of metal having a low barrier height for the underlying semiconductor. Moreover, since one of the main factors on which the specific contact resistance of an M-S junction depends, consists of the concentration of electrically active dopant present in correspondence with the M-S interface, very often the surface of the semiconductor is enriched with dopant before the layers of metal are applied.
The most widely used methods of enrichment consist of predeposition and ion implantation of the dopant. The first method calls for very high temperatures (of over 900.degree. C.).
With a second method, if the ion bombardment has produced a surface layer of amorphous material, it is necessary to carry out the subsequent annealing at a temperature higher than 500.degree.-550.degree. C. in the case of silicon, with a subsequent epitaxial regrowth in the solid phase ("SPE") of the damaged layer and activation of the implanted species. Whereas, if the ion bombardment has not caused amorphization, it is necessary to use a higher annealing temperature to activate the implanted species.
With both the first and the second method, the temperature at which the thermal processes are carried out proves to be harmful for the devices on the front of the wafer.
On the other hand, since it is necessary, upon completion of the front of the wafer, to reduce the thickness of the substrate by means of an appropriate finishing process, the rear surface can only be enriched after completion of the devices present on the front. It is clear therefore that the known techniques for enriching the surface of the semiconductor are of little help in solving this problem.